The present invention relates to a switch mode power supply controller. More particularly, the present invention relates to a pulse frequency modulation switching regulator that deactivates an on-switch of a load circuit for a variable time interval to protect the load circuit from short circuit and overload conditions.
A pulse frequency modulation (PFM) switching regulator regulates a load by activating an on-switch when an output voltage is at or below a voltage regulation setpoint and by deactivating the on-switch after a fixed period of time or when the output voltage exceeds the voltage regulation setpoint. FIG. 1 shows a conventional switching regulator. During normal operation, a PFM controller (CNTR) activates an on-switch (M1) and an input current (IIN) flows into a load circuit (1). The input current (IIN) energizes an inductor (L), and a capacitor (C) is charged to an output voltage (VOUT). The output voltage is provided to a load (ZLOAD) which draws an output current (IOUT). Two resistors (R1, R2) are configured as a voltage divider to sense the output voltage (VOUT). The PFM controller (CNTR) is responsive to a feedback voltage (VFB) from the voltage divider.
An excessive output current (IOUT) can damage the load (ZLOAD). The output current (IOUT) may exceed a current limit due to a short circuit or overload event at the load (ZLOAD). The PFM controller (CNTR) deactivates the on-switch (M1) for a fixed time interval (e.g., 9 microseconds) when the output current (IOUT) exceeds the current limit. A diode (D1) is activated each time the on-switch (M1) is deactivated. The diode (D1) is a freewheeling diode that conducts to provide continuous input current (IIN) to the load circuit (1). Deactivation of the on-switch (M1) causes the energy stored in the inductor (L) to dissipate. The energy dissipation in the inductor (L) causes the input current (IIN) to linearly decrease while the on-switch (M1) is deactivated. The decrease in the input current (IIN) allows the load (ZLOAD) to recover from the short circuit or overload event. The on-switch (M1) activates and the diode (D1) deactivates after the fixed time interval has elapsed.
This summary of the invention section is intended to introduce the reader to aspects of the invention. Particular aspects of the invention are pointed out in other sections herein below, and the invention is set forth in the appended claims, which alone demarcate its scope.
The present invention is directed to an apparatus for providing a limit signal when a current provided to a load circuit by an on-switch exceeds a threshold. The apparatus comprises a current limit time control circuit, a current sense circuit, and a time adjustment circuit. The current limit time control circuit asserts the limit signal for a variable time interval when activated. The current sense circuit activates the current limit time control circuit when the current exceeds the threshold. The time adjustment circuit adjusts the variable time interval in response to an output voltage associated with the load circuit.
Another aspect of the invention is directed to a method for providing a limit signal when a current provided to a load circuit by an on-switch exceeds a threshold. The method comprises sensing the current, activating a time control circuit when the sensed current exceeds the threshold, providing the limit signal for a variable time interval when the time control circuit is activated, and adjusting the variable time interval in response to an output voltage associated with the load circuit.
Still another aspect of the invention is directed to an apparatus for providing a limit signal when a current provided to a load circuit by an on-switch exceeds a threshold. The apparatus comprises a means sensing the current, a means for activating a time control circuit when the sensed current exceeds the threshold, a means for providing the limit signal for a variable time interval when the time control circuit is activated, and means for adjusting the variable time interval in response to an output voltage associated with the load circuit.
A more complete appreciation of the present invention and its improvements can be obtained by reference to the accompanying drawings, which are briefly summarized below, to the following detailed description of illustrative embodiments of the invention, and to the appended claims.